The present invention relates to a dynamic load measuring system and, more particularly, to a method and apparatus for monitoring and measuring the variable dynamic thrust load imposed on a workpiece, such as a valve stem, by the valve operator.
There are many applications in industry where an operating thrust generated to perform useful work must be carefully monitored and controlled within prescribed limits. Thrust producing operators may be of a pneumatic, mechanical, electromechanical or hydraulic type.
The method and apparatus of the present invention are particularly applicable to the monitoring, measurement, and analysis of dynamic valve stem thrust generated in the operation of motor operated valves. Remotely operated valves are well-known in the industry and are commonly used wherever remote operation of a valve is necessary or desirable, such as because of its critical nature or hazardous location. The power generation, chemical and petrochemical industries use motor operated valves extensively and very often on systems wherein the correct operation of the valve is critical to health and safety, as well as to proper operation of the system.
In most cases, a motor-operated valve cannot be installed and set up (or inspected and tested after initial installation) under actual operating conditions where there is fluid flow in the system in which the valve is installed. Thus, motor operated valves are typically adjusted for proper operation, either initially or as a result of periodic maintenance, under static conditions. As is well-known, however, many of the characteristics of valve operation change or vary substantially under dynamic operating conditions. Such variations may be immediately apparent or appear or become aggravated with time and are dependent on such conditions as the actual fluid operating pressure, operator gear train and valve packing wear, variations in voltage supplied to the operator motor, and the nature of periodic maintenance.
A valve operator, typically comprising a motor-driven gear train attached to the valve stem, must impose enough thrust on the valve stem to move the valve disc or plug to a position in which it will stop the dynamic fluid flow through the valve and to seat tightly enough to produce a leak-tight seal. Valve manufacturers typically specify the minimum level of valve stem thrust which must be applied to the stem to properly seat the attached valve disc or plug in the valve seat. Motor operators used with these valves, in turn, include a torque switch which limits the amount of thrust applied to the valve stem by the operator to a level to produce the desired leak-tight seal (generally with an additional margin for safety). An improperly set torque switch can result, if set too low, in failure of the valve to seat and seal properly and, if set too high, in damage to the operator and/or the valve.
A typical valve operator includes a source of motive power, such as an electric motor, which is connected to the valve stem by a power transmission assembly. Basically, the transmission assembly includes a splined shaft (also called the "worm shaft") driven by the motor through a gear reduction assembly. A worm mounted on the worm shaft drives a worm gear which, in turn, turns a drive sleeve containing a stem nut that surrounds a threaded valve stem. The drive sleeve and stem nut are journalled for rotation about the valve stem, but secured within the operator housing against axial movement, such that rotation of the drive sleeve and stem nut result in axial movement of the valve stem to move the valve disc or plug attached to the stem between the open and closed positions. As the valve disc or plug comes into contact with the valve seat (or backseat) as the valve is closed (or opened), an increasing thrust load is imposed on the valve stem which load is transmitted through the transmission assembly to the worm and results in an axial force tending to move the worm along the worm shaft. The mechanical characteristics of a worm/worm gear set are such that the torque applied by the worm gear is directly related to the axial force on the worm. The worm is attached to the operator housing through a spring pack comprising a set of Belleville washers. As the axial stem thrust increases, the axial force on the worm increases proportionally. The Belleville washer spring pack will eventually compress under the axial load and the worm will move axially along the worm shaft.
The operator assembly also includes a torque switch mounted to the operator housing and the switch includes an arm that engages the worm. Axial movement of the worm against the bias of the spring pack will move the torque switch arm and, when the arm has moved a distance established by the adjustable torque switch setting, the torque switch contacts open and stop the motor. The Belleville washer spring pack is attached to the worm in a manner to bias movement of the worm in either axial direction, depending on whether the stem thrust transmitted back to the worm results from valve closure or opening. The torque switch includes switch arms and contacts which are independently adjustable to monitor worm movement and control motor operation in either direction. Settings are commonly called the "close torque switch setting" and "open torque switch setting". The amount of stem thrust available to operate the valve is dependent directly on the torque switch settings.
Motor operators also typically include limit switch assemblies, which may be used in conjunction with or in lieu of the torque switches. The limit switches operate independently and are typically driven by the worm shaft through a gear assembly to stop valve travel after a preset distance. Some of the specific functions of the limit switches are to stop valve travel in the open direction before the valve backseats, bypass the open torque switch while the valve is being unseated, provide remote light indication of valve position, and actuate interlocks with associated equipment.
On motor operators available today, torque and limit switch settings are rather coarse and imprecise. In particular, the construction of a typical torque switch is such that minor changes in the switch setting can result in large changes in stem thrust. Proper adjustment of torque and limit switches is, of course, critically important and has typically required the use of experienced personnel and time-consuming set-and-try procedures. It is now recognized, however, that the most serious basic problem in utilizing torque switch settings based on spring pack displacement is that such displacement is only an indirect indication of the actual thrust on the valve stem. Variations occur through the transmission assembly and valve which affect the theoretical direct relationship between spring pack displacement and stem thrust. These include the preload imposed on the spring pack when the spring pack is assembled, valve packing frictional forces, and the dead weight of the valve stem and plug or disc. In addition, after a motor operated valve has been installed and set, variations occurring over time as a result of wear, maintenance procedures, variations in operating voltage, and the like may eventually result in actual stem thrusts which are below the minimum required to seat the plug and seal the valve or in excess of the maximum which the valve and operator can withstand without damage.
Thus, there has long been a need for a system which can enhance the accuracy of initial motor operated valve set up, permit the actual stem thrusts to be monitored and measured in operation, and permit direct analysis of the function of various valve and operator components for accurate trouble-shooting.
U.S. Pat. No. 4,542,649 (Charbonneau et al) describes a system which is intended to measure valve stem thrust directly and to provide a dynamic trace of the actual stem load throughout the valve operating cycle. The system and related method also monitor motor current and torque and limit switch actuation over the operating cycle and correlate those parameters to the monitored thrust. Output traces of the three parameters are intended to be used to calibrate the operator assembly for initial operation and to monitor the performance of the motor operated valve over time by generating subsequent traces of the three parameters and comparing them with those generated initially.
To provide an indication of actual valve stem thrust, Charbonneau et al disclose an apparatus including a compression load cell operatively attached to the free end of the valve stem (opposite the plug) which provides a direct measurement of the stem thrust load at the end of the valve opening stroke, i.e. at the upper limit of stem travel. Stated another way, the Charbonneau et al apparatus can measure only the actual stem thrust at which the open torque switch trips. This single measurement of stem thrust is then used to establish both the open and close torque switch settings. The thrust measurement is also utilized to calibrate the linear displacement of the spring pack which is measured directly to generate a thrust trace over valve cycle time intended to be representative of the actual stem thrust. Thus, the apparatus of Charbonneau et al is incapable of monitoring and measuring actual stem thrust directly over the entire valve operating cycle and, as mentioned previously, reliance on spring pack displacement as an indication of actual stem thrust gives rise to a number of inaccuracies. In addition, because this apparatus is incapable of measuring actual stem thrust on valve closure, certain assumptions rust be made in order to use the measured valve opening stem thrust to establish the close torque switch setting. The most important assumption is that the torque (and stem thrust) to open the valve is equal to the torque required to close the valve, i.e., it is assumed that spring pack displacement for a given opening thrust is exactly the same as spring pack displacement for an equal closing thrust. This assumption clearly ignores differences attributable to the weight of the valve plug and stem, variations in the valve packing friction with direction, and variations in transmission assembly efficiency because valve opening and closing utilizes two different faces of the gear teeth.
Also, the Charbonneau et al system utilizes a measurement of current as the indicator of torque and limit switch actuation and, therefore, valve position. This requires a series connection to the torque and limit switches and, therefore, a break in the switch connections. In the nuclear industry, where large numbers of motor operated valves are used, a series connection requiring a break in the lead requires double verification of return to service which is costly and time-consuming.
U.S. Pat. No. 4,570,903 (Crass) also addresses the problem of direct measurement of valve stem thrust to allow accurate setting of the close torque switch, both initially and as a result of thrust variations related to system operating changes. The apparatus of Crass allows the direct measurement of valve stem thrust in a manner somewhat like Charbonneau et al. Like Charbonneau et al, however, only a single direct measurement of stem thrust can be made over the valve operating cycle. A load cell is operatively attached to the free upper end of the valve stem and is disposed to measure actual stem thrust only at the end of the valve closing stroke. The apparatus measures the thrust necessary to trip and deactivate the close torque switch. However, the apparatus cannot monitor and measure the actual valve seating thrust which may be substantially higher than the thrust necessary to trip the torque switch because of gear train inertia, delays in motor contactor drop-out, etc.
There is, therefore, presently no apparatus or method which can monitor actual valve stem thrust continuously and dynamically over the full operating cycle of the valve, from fully open to fully closed in both directions. Prior art apparatus and methods which provide capability for measuring actual valve stem load are limited in their ability to measure only a single such load in the valve operating cycle. Further, such a single thrust load measurement may not be the actual maximum thrust occurring at that point in the valve cycle. Where the prior art discloses the measurement of an indication of valve stem load over the entire operating cycle, such measurement is in fact only indirect and, as a result, not an accurate indication of actual valve stem thrusts.